Cathode-ray tube focusing circuit



Jan. 11,1949. BOYLE 2,458,891

GATHODE-RAY TUBE FQCUSING CIRCUIT 7 Filed Jan. 11; 1947 i 71 1. v 11 Z6 v| DEO I AMPLIFIER STAGE nil- V VIDEO -a AMPLIFIER STAGE nil- 61 5mm 6.30m

INVENTOR AGENT Patented Jan. 11, 1949 UHTED. S,

CATHODE-RAY TUBE FOCUSING CIRCUIT of Delaware Application January 11, 1947, Serial No 721,553

6 Claims.

The. invention relates to circuit arrangements for cathode ray tubes and'particularly pertains to circuit arrangements forensuring stability of the focusing of cathode ray tubes; throughout variations of theoperating potentials thereof.

In employing cathode ray tubes, 1, e. television receiving apparatus, it has been found that the imageproduced; on; the screen of the cathode ray tube. becomes distorted due to defocusing of the cathode ray beam with a chang in. the demand made-upon the powersupply thereof by the complex and abruptly. changing signal received. While this defocusingeffect may in part be correctedbythe. use of a power supplyof good voltage regulation, i. e. by the use of a heavy duty power supply, this solution to the problem isnot a satisfactory one because of the. inherent weight and bulk and attendant high cost of such a power supply.

It is an object of the invention to provide a circuit arrangement whereby defocusing of the cathode ray beam in a television receiver is substantially eliminated.

It is another object of the invention to provide a circuit arrangement for stably maintaining the focus of a cathode ray beam in equipment embodying a power supply of poorregulation,

It is a further-object of the invention to provide a circuit arrangement to eliminate defocusing of the cathode ray beam which is applicable to inexpensive television receivers,

These and other objects of the invention will appear as the specification progresses.

According to the invention, changes in the focus of a cathode ray tube due to variations in its operating potential brought. about by variations in the demands made on the power supply by changes in the intensity of the beam current are corrected by adjusting the focusing potential proportionally to the voltage which produces the changes: in the. cathode ray beam intensity.

More particularly, in an image system for television receivers, the focusing potential" of the cathode ray tube is made proportional tothe video signal applied to the beam intensity controlling electrode of the cathode ray tube.

The invention will be described in greater detail with reference to the accompanying drawing forming a part of the specification and which:

Fig. 1 is a schematic diagram of a circuit arrangement according to the invention as applied to an electrostatically focused cathode ray tube, and

Fig. 2 is a schematic diagram of a circuit arrangement according to the invention as applied to an. electromagnetically. focused cathode ray tube.

Referring to Fig. 1 there is shown an electrostatically focused cathode ray tube l l, which come prises a cathode [2, a grid, [3, a focusing electrode M, an accelerating anode l5 and a luminescent screen [6. The various potentialsirequired for energizin-gthe tube areprovided by tapping across a resistor network comprising resistors l1, l8, I9, 29, 2| and..22 connected, in series across a source of direct current potential (not shown) one end of resistor I! being connected to the negative terminal of said source and the remote end of resistor 22 being connected to, the positive terminal. 'Ifhe interconnection of resistors I! and I8 is connected to ground. Cathode I2 is connected to the interconnection of resistors l8. and I9. Grid His biased by means of resistor 23 connected to an, adjustable, tap 24 on resistor l8. Focusing electrode IA is nergized by means of an adjustable tap 2 5 on resistor 21. The accelerating anode I5 is connected directly to the positive terminal of the potential source. Suitable means to deflect thev cathode ray beam are pro,- vided in the form of coils, 26 and 26' which are coupled to circuitsv suitable for controlling. the beam in a predeterminedmanner. These circuits may be entirely conventional and are therefore not shown. For simplicity only one set of coils is shown. Operatin voltages to intensity modulate the cathode ray beam are provided by coupling grid I3 by means of a capacitor 21 to the output circuit of a video amplifier 28 which is shown in block form. Video amplifier 28 is entirely conventional and is energized by potential ob tained from the interconnection of resistors l8 and l9. In operation, adjustable tap 24 on resistor i8 is set to provide the desired degree of modulation and th potential on focusing electrode It is set by means, of tap 25 on resistor 2| to provide the desired definition on screen l6. An abrupt change of signal voltage produces an abrupt, demand on the potential supply source which adversely afifects the potential on the focusing electrode 1 For example, an increase of video frequency voltage from amplifier 28 makes grid l3 more positive with respect to cathode l2, causing an increase in beam current, which in turn imposes an increased current demand on the potential source. This increased demand brings about a lowering of potential available due to the increased potential drop in the power supply circult, which effect is well known to the art.

The lowering of available potential also lowers the focusing potential, but since the focusing electrode draws little current the ratio of focusing potential to accelerating and potential is changed thereby causing the spot displayed on screen I6 to lose its normal definition and become large and fuzzy, an action which isusually termed blooming by those familiar with the art. In

the circuit arrangement shown in Fig. 1, however, 7

blooming is eliminated by the action of an electron discharge tube 29, which is preferably a pentode but which may be of almost any other type of grid controlled tube. Tube 29 comprises a cathode 30 connected to ground by means of resistor 3i, a grid 32 connected to the adjustable tap 33 of a resistor 34, one end of which is connected to an adjustable tap 35 on resistor I! thereby providing bias for grid 32, screen grid 36 energized by potential-taken from the interconnection of resistor I9 and 2D, suppressor grid 31 connected to cathode 3H, and an anode 38 connected to the cathode ray tube focusing electrode I4 and energized by the potential applied thereto. Capacitors 4!] and M connecting screen 36 and anode 38 respectively to ground serves to bi-pass video frequency currents to ground; capacitor 4! being of low capacity in order that video frequency currents are not unduly suppressed and prevented from affecting the focus of the cathode ray beam.

The end of resistor 3 3 remote from tap 35 is coupled by means of a capacitor 39 to the output circuit of video amplifier 2B in any known manner so that the voltage applied to grid 32 of tube 29 is in the proper phase relationship with that applied to grid I3 of cathode ray tube II. Hence, for an increase in voltage on cathode ray tube grid I3, an anti-phase input voltage to tube 29 makes grid 32 less positive with respect to cathode 30, thereby reducing the current flow in the anode circuit and increasing the potential across the By adjusting the input voltage and the bias on tube 29 by means of adjustable taps 33 and 35 respectively, the potential applied to focusing electrode I l can be maintained at the value corresponding to correct focus of the cathode ray beam. 7

Referring to Fig. 2, there is shown an electromagnetically focused cathode ray tube 5|, which comprises a cathode 52, a grid 53, an accelerating anode 54 and a luminescent screen 55. Mounted externally to and surrounding the neck of the cathode ray tube is a solenoid 56 which forms the focusing element for cathode ray tube 5|. The various potentials required for energizing. the tube are provided by tapping across a resistor network comprising resistors 51, 58, 59 and 60 connected in series a cross a source of direct current low potential (not shown) one end of resistor 51 being connected to the negative terminal of said source and the remote end of resistor'fifl being connected to the positive terminal thereof.' The interconnection of resistors 51 and 58 is connected to ground. Cathode 52 is connected to the interconnection of resistors 58 and 59. Grid 53 is biased by means of resistor 62 .connected to an adjustable tap 63' on resistor 58. The accelerating anode 54 is connected directly to the positive terminal of a high potential source (not shown), which source is shunted by a bleeder resistor 6|. As in the arrangement shown in Fig. 1 means for defleeting the cathode ray beam are shown in the form of coils 26 and 26' which are coupled to circuits suitablefor controlling the beam in desired manner. Focusing coil 56 is energized by means of this circuit comprising a variable resistor 6d connected directly to the low potential source and :an electron discharge tube 65 whose cathode 66 is connected to ground by means of resistor El. Tube 55 comprises cathode 66, previously mentioned, control grid 68, screen grid 59 energized by potential taken from the interconnection of resistors 59 and 60, suppressor grid 1U connected to the cathode 55, and anode H directly connected to focusing coil 55. Grid 53 is biased by means of resistor 62 connected to an adjustable tap 63 on resistor-58. Operating voltages to intensity modulate the cathode ray beam are provided by coupling said grid by means of capacitor T2 to a video amplifier 28, which amplifier is identical to that shown in the arrangement of Fig. 1. Video amplifier 28 is en-ergizedby potential obtained from the interconnection of resistors 58 and 59. Grid 68 of tube 65 is connected to an adjustable top I4 on a resistor 15, one end of which is connected to an adjustable bias for said tube. The end of resistor 15 remote from tap I6 is coupled by means of capacitor 11 to the output circuit of video amplifier 28 in the same manner as described in the foregoing paragraphs relating to the embodiment illustrated in Fig. 1. As in the previously described embodiment capacitors MI and ll connecting screen 69 and anode II respectively serve -to bi-pass video frequency currents to ground.

The operation of the circuit arrangement shown in Fig. 2 is similar tothat of circuit arrangement shown in Fig. 1 except that the correction of the focus of the cathode ray tube is obtained by the change of current flowing through coil 56. By adjusting the input voltage and the bias on tube 55 by means of adjustable taps I4 and 16 respectively, the current passing through focusing coil 55 can be maintained at the value corresponding to correct focus of the cathode ray beam.

While, for the purpose of simplification the invention has been described as applied to circuit having but one source of potential applied across a resistor network, it is to be understood that any of the various power supply arrangements known to the art may be used with equal success, and it is to be further understood that the invention is not 1imited to those specific embodiments described but that various modifications may be made in the invention without departing from one spirit and scope thereof.

I claim:

1. An electronic circuit arrangement comprising a cathode ray tube having an electron beam generating cathode electrode, a grid electrode,

' an anode electrode and a focusing element, a direct current potential source connected between said cathode electrode and said anode electrode, means to apply a first voltage of variable amplitude to the grid electrode of said cathode ray .tube to intensity modulate the electron beam portional to changes in the intensity of said beam and thereby normally producea defocusing of said beam, an electron discharge tube having a greater said first variable amplitu'devoltage to the corn trol electrode of said discharge tube, and means to couple the anode-cathode circuit of said dischargetube .to said focusing element to thereby maintain said electron beam in'focus independently of said voltage fluctuations of said potential source.

2. An electronic circuit arrangement comprising a cathode ray tube having an electron beam 2 generating cathode electrode, a grid electrode, an anode electrode and a focusing element, a direct current potential source connected between i said cathode electrode and said anode electrode, means to apply a first voltage of variable amplitude to the grid electrode of said cathode ray tube to intensity modulate the electron beam thereof, said potential source being responsive to and thereby normally produce a defocusing of said beam, an electron discharge tube having a cathode electrode, a control electrode and an anode electrode, means to apply operating potentials to the electrodes of said discharge tube, means to apply a second voltage inversely proportional to said first variable amplitude voltage to the control electrode of said discharge tube, and means to couple the anode-cathode circuit of said discharge tube to said focusing element to thereby maintain said electron beam in focus independently of said voltage fluctuations of said potential source.

3. An electronic circuit arrangement comprising a cathode ray tube having an electron beam generating cathode electrode, a grid electrode, an anode electrode and a focusing electrode, a direct current potential source connected between said cathode electrode and said anode electrode, means to apply a first voltage of variable amplitude to the grid electrode of said cathode ray tube to intensity modulate the electron beam thereof, said potential source being responsive to the modulation of said beam to produce in the output thereof voltage fluctuations inversely proportional to changes in the intensity of said beam and thereby normally produce a defocusing of said beam, means to apply a potential to said focusing electrode to thereby produce focusing of said beam at a given current intensity thereof, an electron discharge tube having a cathode electrode, a control electrode and an anode electrode, means to apply operating potentials to the electrodes of said discharge tube, means to apply a second voltage proportional to said first variable amplitude voltage to the control electrode of said discharge tube, and means to couple the anodecathode circuit of said discharge tube to said focusing electrode to vary the potential thereof proportionally to the current intensityof said beam and thereby maintain said electron beam in focus independently of said voltage fluctuations of said potential source.

4. An electronic circuit arrangement-comprising a cathode ray tube having anelectron beam" generating cathode electrode, a grid electrode, an anode electrode and a focusing coil, a direct current potential source connected between said cathode electrode and said anode electrode, means to apply a first voltage of variable amplitude to the grid electrode of said cathode ray tube to intensity modulate the electron beam thereof, said potential source being responsive to the modulation of said beam to produce in the output thereof voltage fluctuations inversely proportional to changes in the intensity of said beam and thereby. normally produce. a defocusing of said beam, means to apply a potential across said focusing coil to thereby produce focusing of said beam at a given current intensity thereof, an

electron discharge tube having a cathode electrode, a control electrode and an anode electrode, means to apply operating potentials to the electrodes of said discharge tube, means to apply a second voltage proportional to said first variable amplitude voltage to the control electrode of said discharge tube, and means to couple the anodecathode circuit of said discharge tube to said focusing coil to vary the current flowing therethrough proportionally to the current intensity of said beam and thereby maintain said electron beam in focus independently of said voltage fluctuations of said potential source.

5. An electronic circuit arrangement comprising a cathode ray tube having an electron beam generating cathode electrode, a grid electrode, an anode electrode and a focusing electrode, a direct current potential source, a series resistor network connected across said source, means to connect said cathode electrode of said cathode ray tube to said resistor network, means to connect said anode of said cathode ray tube to the positive pole of said source, means comprising an adjustable tap on said resistor network to bias the grid electrode of said cathode ray tube, means comprising an adjustable tap on said network to apply a potential to said focusing electrode to thereby produce focusing of said beam at a given intensity thereof, an electron discharge tube having a cathode, a control grid and an anode, means comprising a tap on said resistor network to apply a biasing potential to said control grid of said discharge tube, means to connect said anode of said discharge tube to said focusing electrode, means to apply a first voltage of variable amplitude to the grid electrode of said cathode ray tube to intensity modulate the electron beam thereof, said potential source being responsive to the modulation of said beam to produce in the output thereof voltage fluctuations inversely proportional to changes in the intensity of said beam and thereby normally produce a defocusing of beam, and means to apply a second voltage inversely proportional to said first variable amplitude voltage to the control grid of said discharge tube to thereby maintain said electron beam in focus independently of said voltage fluctuations of said potential source.

6. An electronic circuit arrangement comprising a cathode ray tube having an electron beam generating cathode, a grid electrode, an anode electrode and a focusing coil, means to connect a direct current potential source between said oath ode electrode and said anode electrode, means to energize said focusing coil, an electron discharge tube having a cathode, a control grid and an anode connected in series with said coil and said coil energizing means to apply a biasing potential to said control grid to vary the steady state current through said coil, means to apply a first voltage of variable amplitude to the grid of said cathode ray tube to intensity modulate the electron beam thereof, said potential source being responsive to the modulation of said beam to produce in the output thereof voltage fluctuations inversely proportional to changes in the intensity of said HOMER G. BOYLE.

8 v REFERENCES CITED The following references are of recordinthe file 'of this patent:

' UNITED STATES PATENTS Number Name Date 2,116,671 Dowsett et a1. May 10, 1938 2,255,485 Dome Sept. 9, 1941 2,291,682 Blumlein et a1; Aug. 4, 1942 

